Realm Functions allow you to define and execute server-side logic for your application. You can call functions from your client applications as well as from other functions and in JSON expressions throughout Realm.

Functions are written in modern JavaScript (ES6+) and execute in a serverless manner. When you call a function, you can dynamically access components of the current application as well as information about the request to execute the function and the logged in user that sent the request.

By default, Realm Functions have no Node.js modules available for import. If you would like to make use of any such modules, you can upload external dependencies to make them available to import into your Realm Functions.

Use Cases

You can use functions to handle low-latency, short-running connection logic and other server-side interactions. Functions are particularly useful when you want to work with multiple services, behave dynamically based on the current user, or abstract away implementation details from your client applications. You can upload external Node.js packages and import them into your functions to streamline data processing and reuse shared code.

Realm also uses functions internally for incoming webhooks and triggers. The functions that you create for these components are regular Realm functions that take specific arguments depending on the service.


  • Realm automatically encodes values returned from functions as Extended JSON.
  • Functions do not continue to execute after they return. You cannot use functions with asynchronous callbacks or event listeners. If you need to do asynchronous work in a function, use a Promise.


  • Function runtime is limited to 90 seconds.
  • Function memory usage is limited to 256MB.
  • Functions support most commonly used ES6+ features, but some features that are uncommon or unsuited to serverless workloads are not supported. For more information, see JavaScript Feature Compatibility.
  • A function may open a maximum of 5 sockets using the net built-in module.
  • Realm supports a subset of built-in Node modules. For a full list of supported and unsupported modules, see Built-In Module Support.
  • There is an 18 MB limit for incoming webhook and function requests. For functions called from an SDK, this limit applies to the total size of all arguments you pass to the function.


Active User

The active user of a given function execution is the authenticated user that called the function. Functions inherit the user context of other functions that call them.

MongoDB Realm evaluates rules in the context of the active user and resolves dynamic references like context.user and %%user to their user object.

System Functions

A System Function is a function that runs as the system user rather than an application user. System functions have full access to MongoDB CRUD and Aggregation APIs bypass collection rules and schema validation.

You can configure a Function to run as the system user by enabling its Run As System configuration option. To determine if a function executes as a system user at runtime, call context.runningAsSystem().

System Function Active User

Realm always resolves dynamic references like context.user and %%user to the authenticated user that called a function if there was one. If a function executes without being called by an authenticated user, such as in a trigger or webhook, then Realm resolves the dynamic references to the system user instead.

External Dependencies

An external dependency is an external library that includes logic you’d rather not implement yourself, such as string parsing, convenience functions for array manipulations, and data structure or algorithm implementations.

Realm allows you to upload external dependencies from the npm registry and then import those dependencies into your functions using standard JavaScript module syntax. Realm automatically transpiles dependencies and supports most built-in Node.js modules. To get started with external dependencies, check out the following guides:

Create Your Own Modules

Though most npm modules are written by third parties, you can also create and publish your own npm modules to house logic specific to your application. You can make your modules available to the Node.js community or reserve them for private use. For more information, check out npm’s guide on Contributing packages to the registry.


Guide Description
Define a Function Learn how to write and add new functions to your Realm app.
Upload an External Dependency Learn how to make Node.js packages available for import in your Realm Functions.
Call a Function Learn how to execute functions that you’ve defined from a client application or from another Realm component.
Import an External Dependency Learn how to import and call uploaded Node.js packages within a Realm Function.
Access Function Context Learn how to access runtime information and interfaces to other components of your application from within a function.

Reference Documentation

Subject Description
Function Context Includes detailed information about specific application context modules as well as examples of runtime data objects.
Utility Packages Includes detailed information about global utility packages.
JSON & BSON Details built-in global modules for working with JavaScript and MongoDB data formats.
Built-In Module Support Details which standard Node modules and APIs are available in Realm functions.
JavaScript Feature Compatibility Details the JavaScript syntax and built-ins that are available in Realm functions.